The invention relates to an apparatus for heat treating fine-grain material, in particular for the production of cement, comprising a rotary kiln, a preheater, a kiln waste gas fan, and electrofilter serving to remove dust from the kiln waste gases, and a bypass system comprising a bypass conduit connected between the rotary kiln and preheater and an apparatus section serving to dedust the bypass flow.
Apparatuses of this type (see "Zement-Kalk-Gips", 1962, page 203, FIG. 11) are used when the raw material has such a high content of noxious constituents (e.g. alkalis) that by circulation of these noxious substances within the apparatus an inadmissibly high concentration of noxious substances could arise. By branching off a part of the kiln exhaust or waste gases and separate dedusting of said bypass flow (the dust separating from the bypass flow not being returned to the system) the formation of a troublesome circulation of noxious substances (for example alkali circulation) can be avoided.
In the known apparatuses, the bypass system generally comprises either an electrofilter and a preceding cooler or a cyclone for dedusting the bypass flow. The kiln waste-gases conveyed via the waste gas fan generally are supplied to a raw meal grinding apparatus and thereafter dedusted by an electrofilter. When the mill is not operating the kiln waste gases can also be supplied directly to the electrofilter after the necessary cooling.
The bypass system necessary to reduce the concentration of noxious substances involves a considerable expenditure on apparatus. This expenditure is undesirable, particularly in cases in which high contents of noxious substances in the raw material requiring the bypass system to be switched on occur for only short times and only at relatively long intervals.